The invention relates to sports equipment for ball games comprising a stroke portion and a shaft at the external end of which a handle portion is provided, particularly to tennis rackets and golf clubs. It can however also be used for sports equipment for other ball games where the ball is struck with a high force, as in cricket, polo, baseball etc.
It has been known for a long time that the "tennis elbow" so dreaded by all tennis players and golf players is caused in the first instance by the vibration or after-oscillation of the racket or club during the hitting action. For this reason, manufacturers of tennis rackets all over the world have been searching for constructional possibilities to attenuate these vibrations of the racket. The same is true for other sports equipment by which a ball has to be hit with great force by a player, for instance a golf club (wood 1-3). While the clubs or rackets are differently formed according to the type of sport, they have in common that the player holds the equipment at a handle portion which is normally at the external end of a shaft, and via a more or less extended shaft transfers his high stroke force via the stroke portion of the equipment onto the ball. The kick-back and the oscillation of the equipment fully react on the player's arm. By the kick-back and the natural vibration of the racket, further on, striking energy is lost which could be used for the acceleration of the ball.
Latest developments on the field of sports equipment, particularly tennis rackets and golf clubs, show that the aim of obtaining a real freedom from vibration and a good shock absorption and a good oscillation attenuation has a yet not nearly been reached.
It has already been known that-by integration of a freely movable mass, for instance lea shot or a heavy liquid, into a tennis racket, oscillation attenuation can be obtained. This effect has for instance been used in U.S. Pat. No. 4,812,512 describing a tennis racket of the earlier design in that additional chambers were applied outwardly to the finished racket. By such a supplementary application, however, the racket becomes heavy and tends to nose heaviness. In a further embodiment of the cited U.S. Patent, a subdivision of the hollow interior space of the racket frame is provided in the kind of compartments whereby such a subdivision chambers are produced in the interior space of the frame which are each filled with the freely movable mass particles, for instance lead shot. In this embodiment, the chambers are relatively large as compared to the interior space of the frame. By too large individual masses however, an insufficient oscillation attenuation is obtained since at oscillation the mass particles are to a great extent at an oscillation nodal point or at zero passage so that attenuation is not at its optimum.
There is still a need to further improve the attenuation values of tennis rackets. This is particularly true for the attenuation of the stroke shock. This strain is the most dangerous for the tennis player's wrist.
An important role with the tennis rocket-further on plays the "sweetspot" which corresponds to the area which is defined by the zones in which the attenuation starts to drop relatively steeply from a high value to low values. The positioning of the "sweetspot" on the tennis racket area has been the object of many endeavors and calculations. Since, however, different stroke characteristics also imply different positions of the sweetpoint, a general optimization of the position of the sweetpoint, small in relation to the racket area, was not possible.
With respect to golf clubs, the present inventor has carried out several studies and experimentations which have shown that the simple theory of the stroke between the golf club head and the ball is not accurate. That is, it has been believed that the momentum during the process of the ball and the head contact is constant. The inventor, in fact, has found that a theory of the ball and head contact includes not only one momentum but two momentums of different value.